This invention pertains to a means for controlling the carrier phase modulation amplitude of a coherent optical signal output of an optical interferometer sensor system which employs a coherent optical source such as a solid state laser.
The interferometer sensor system may be fabricated using optical fibers and couplers, but the invention is not limited to such an arrangement. For example, the carrier phase modulation for a bulk optic interferometer configuration or an electrical mixing system can be controlled by the technique of this invention.
The quantity which is to be measured is called herein the "physical quantity." It includes, in part, such quantities as acceleration, pressure, magnetic field intensity, temperature and sonic waves. The apparatus may be used, for example, as a fluid pressure sensor in open water.
An example of such a sensor is described in an article, "Homodyne Demodulation Scheme for Fiber optic Sensors Using Phase Generated Carrier" by Anthony Dandridge, Alan B. Tveten, and Thomas G. Giallorenzi.which was published in the IEEE Journal of Quantum Electronics, Volume QE-18, No. 10, October 1982.
In a first topology, an optical source, such as a laser drives an interferometer. The optical source is frequency modulated to provide an input to the arms of the interferometer from a coupler. The interferometer fiber arms' lengths might be typically two or three centimeters. The actual difference in length to be used will depend on the particular design application. The actual difference in length will be functionally related to the maximum frequency excursion of the optical source as it is driven through a frequency excursion by the modulator.
As an alternative to frequency modulating the optical source, a phase modulator can be interposed in one of the arms of the interferometer and driven by the modulator source. A modulator of this type might typically comprise a PZT bobbin on which several turns of fiber from the respective arm of the interferometer is wound. The modulator voltage applied to the PZT increases and decreases the circumference of the PZT (fiber spool on the bobbin) thereby increasing and decreasing the respective optical path length of the arm.
The light exiting the two arms is combined in a second coupler, and the combined light from the second coupler is then focused on a photodetector. The combined light exiting the second coupler is the summation of the light from the first and second arms, each of which is being phase modulated, with respect to each other, as a result of the frequency modulation imposed on the optical source and the difference in lengths of the respective arms of the interferometer.
The fibers may be extended or they may be otherwise configured. For example, they could be coiled and potted. The length of each fiber can be made sensitive to changes in the measured physical quantity; or it can be made substantially insensitive to such physical quantity. The apparatus may be operated with one fiber sensitive and the other fiber insensitive to the physical quantity to be measured (such as pressure); or it may be made with both fibers sensitive to the physical quantity to be measured, with the two fibers connected in a push-pull arrangement wherein one fiber length increases while the other decreases for a given change in the physical quantity. The sensitivities of the two fibers may differ.
The measured physical quantity may be "pressure" and may be so-described herein with the understanding that any other quantity which can affect the fiber length may similarly be measured.
Although the invention is described as controlling the alternating component of current delivered to a laser diode, it is possible that a different coherent optical source could be used, and it could be frequency modulated by controlling a parameter different from the delivered current. For example, the optical source output could be passed through an integrated-optic phase modulator to provide the optical frequency modulation. (The optical frequency change is the time derivative of the optical phase change).
It is intended that this invention include control of other parameters by calling the modulation current more generally the "modulation factor" and by calling the laser diode a "coherent optical source".
The quiescent phase difference of the interferometer can drift many (even tens of thousands) of wavelengths due to pressure and temperature changes. The phase differences in the detected signal caused by slowly changing acoustic pressure or temperature can usually be separated from phase differences in the detected signal resulting from the sensed physical quantity because the phase difference in the detected signal resulting from the sensed physical quantity occur at a sufficiently higher frequency than the frequency of the phase difference in the detected signal resulting from drifting pressure and temperature variations.
Changes in fiber length difference between the arms of the interferometer change the phase, .phi., between the interfering beams, producing signal amplitudes that are unipolar sinusoidal functions of the phase difference.
A change in fiber length produces a much smaller change in instantaneous phase amplitude when operating at an average phase corresponding to the peak or trough of a sine wave than it produces when operating at the maximum sine wave slope. To make the apparatus more stable, i.e. less sensitive to quiescent phase shift or change between the two light beams, the optical source is frequency modulated at a frequency that is more than twice the highest phase modulation frequency component produced by the expected sensed physical parameter. In a preferred example, the modulation frequency is on the order of 10 to 100 khz. When a frequency modulated optical signal is input to an unbalanced path length interferometer, the phase difference between the interfering beams is phase modulated. The phase modulated signal may be considered a carrier for the signal produced by changes in the measured physical quantity, and the signal phase difference is superimposed upon the oscillating phase modulated signal.
The phase modulation amplitude between the optical signal exiting one arm with respect to the optical signal exiting the second arm is contained in and characterized by the amplitudes of the harmonics in the detected output signal and the ratios of the amplitudes of the harmonics in the detected output signals.
The relative amplitudes of the harmonics or the ratios of the amplitudes of the harmonics in the detected output signal inherently correspond with Bessel's Functions of the first kind having an argument (X) equal to the peak carrier phase modulation (measured in radians) between the interfering light beams at the detector. The odd and even frequency harmonics in the interferometer photodetected signal are in quadrature. The odds (1,3, . . . ) are in quadrature with the evens (0,2, . . . )at all times. By way of clarification, the first odd harmonic corresponds in amplitude to the J.sub.1 (X) coefficient. The amplitude of the second odd harmonic corresponds to the J.sub.3 (X) coefficient. Excluding the dc or J.sub.0 (X) for this application, the first even harmonic amplitude corresponds to the J.sub.2 (X) term. The Bessel Coefficients corresponding to the required values are available in published tables.
The output optical signal of a laser diode may be frequency modulated by changing the current delivered to the diode. The current delivered to the laser diode has both a d.c. component and an a.c. component. The a.c. component produces the phase modulation in the interferometer output, and it is the amplitude of the current delivered to the laser diode that is controlled by the apparatus of this invention.
The amplitude of the phase modulation of the interfering optical beams is controlled to control the sensitivity or scale factor of the demodulated output relative to a change in the phase angle between the interfering beams. As the amplitude of the ac component of the current modulating the laser frequency varies, the optical frequency of the laser output varies. As stated above, the detected interferometer output signal has a frequency spectrum wherein the coefficients of the various frequency terms are Bessel's Functions of the first kind.
The a.c. current for the laser diode is its modulation factor, and the invention will be described as controlling that current. The amplitude of that current or factor is substantially sinusoidal. Bessel's functions of the first kind of an argument, "x" appear in the coefficient expressions for the sine and cosine terms of the interferometer output phase angle, .phi.. The argument, "x" corresponds to the interferometer peak phase modulation in radians. The interferometer output signal can be sensed and its spectrum analyzed. When analyzed, it is found that as the length difference of the two fibers changes, the frequencies corresponding to the even and odd carrier multiples fade in quadrature so that when the amplitudes of the even carrier multiples fade out, the odd carrier multiples peak, and vice versa.
The amplitudes of the various Bessel's functions are known for the different values of the argument, X. Therefore, the ratios of the variously numbered Bessel's functions to each other are also known for each value of their argument, x. According to this invention, the ratios of the coefficient amplitudes of the various frequency terms sensed by the sensor are controlled to conform to the known ratios of the Bessel's Functions. For convenience, values of x wherein J.sub.1 (x) should equal J.sub.2 (x), (x=2.63), and wherein J.sub.2 (x) should equal J.sub.3 (x), (x=3.77), are chosen for controlled amplitudes of the value of x. The modulation factor is driven at an amplitude to produce the desired Bessel function amplitude ratios of the coefficients of the carrier harmonics for the chosen value or x. Previously, an operator monitored the detected interferometer output carrier signal levels (corresponding to the selected Bessel orders) on a spectrum analyzer while the interferometer phase slowly drifted. The operator adjusted the amplitude of the current to the laser optical source to cause the coefficient of the peaked value of the carrier frequency component, fo, in the interferometer output signal to equal the peaked value of the coefficient of the 2fo frequency component when the desired operating peak phase modulation (x) was 2.63 radians. If the selected operating peak phase modulation was 3.77 radians, the operator adjusted the laser a.c. driving current to equalize the peak output signal amplitudes of the quadrature components at the frequencies 2fo and 3fo.